The iBiquity Digital Corporation HD Radio™ system is designed to permit a smooth evolution from current analog amplitude modulation (AM) and frequency modulation (FM) radio to a fully digital in-band on-channel (IBOC) system. This system delivers digital audio and data services to mobile, portable, and fixed receivers from terrestrial transmitters in the existing medium frequency (MF) and very high frequency (VHF) radio bands. Broadcasters may continue to transmit analog AM and FM simultaneously with the new, higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog to digital radio while maintaining their current frequency allocations.
The design provides a flexible means of transitioning to a digital broadcast system by providing three new waveform types: Hybrid, Extended Hybrid, and All Digital. The Hybrid and Extended Hybrid types retain the analog FM signal, while the All Digital type does not. All three waveform types conform to the currently allocated spectral emissions mask.
The digital signal is modulated using Orthogonal Frequency Division Multiplexing (OFDM). OFDM is a parallel modulation scheme in which the data stream modulates a large number of orthogonal sub-carriers, which are transmitted simultaneously. OFDM is inherently flexible, readily allowing the mapping of logical channels to different groups of sub-carriers.
The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an IBOC standard, designated NRSC-5A, in September 2005. NRSC-5A, and its update NRSC-5B, the disclosure of which are incorporated herein by reference, sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. The standard and its reference documents contain detailed explanations of the RF/transmission subsystem and the transport and service multiplex subsystems. Copies of the standard can be obtained from the NRSC at http://www.nrscstandards.org/SG.asp. iBiquity's HD Radio™ technology is an implementation of the NRSC-5 IBOC standard. Further information regarding HD Radio™ technology can be found at www.hdradio.com and www.ibiquity.com.
A typical HD Radio broadcast implementation partitions content aggregation and the audio codec into what is typically referred to as an exporter. An exporter will typically handle the sourcing and audio coding of the Main Program Service (MPS), that is, the digital audio that is mirrored on the analog channel. Feeding into the exporter may be an importer, which aggregates secondary programming other than MPS. The exporter then produces over-the-air packets and forwards those to an exciter or modem part of an exciter platform, which is typically referred to as the exgine.
In some instances, it would be desirable to implement an HD Radio broadcast system as a single frequency network (SFN). Generally, a single frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel. Analog FM and AM radio broadcast networks, as well as digital broadcast networks, can operate in this manner. One aim of SFNs is to increase the coverage area and/or decrease the outage probability, since the total received signal strength may increase at positions where coverage losses due to terrain and/or shadowing are severe.
Another aim of SFNs is efficient utilization of the radio spectrum, allowing a higher number of radio programs in comparison to traditional multi-frequency network (MFN) transmission, which utilizes different transmitting frequencies in each service area. In MFNs, hundreds of stations are established for a national broadcasting service; therefore many more frequencies are used. Simultaneous transmission of programming on multiple frequencies can be confusing to listeners who often don't remember to retune their radios when traveling between coverage areas.
A simplified form of SFN can be achieved by a low power co-channel repeater or booster, which is utilized as a gap filler transmitter. In the United States, FM boosters and translators are a special class of FM stations that receive the signals of a full service FM station and transmit or retransmit those signals to areas that would otherwise not receive satisfactory service from the main signal, again due to terrain or other factors. Originally, FM boosters were translators on the same frequency of the main station. Prior to 1987 FM boosters were limited, by the FCC, to using direct off-air reception and retransmission methods (i.e., repeaters). An FCC rule change allowed the use of virtually any signal delivery method as well as power levels up to 20% of the maximum permissible effective radiated power of the full service station they rebroadcast. With this rule change, FM boosters are now essentially a subclass of SFNs. Many domestic broadcasters currently make use of FM boosters to fill in or extent coverage areas, especially in hilly terrains such as San Francisco.
In areas of overlapping coverage, SFN transmission can be considered as a severe form of multipath propagation. A radio receiver receives several echoes of the same signal, and the constructive or destructive interference among these echoes (also known as self-interference) may result in fading. This is problematic since the fading is frequency-selective (as opposed to flat fading), and since the time spreading of the echoes may result in inter-symbol interference (ISI).
When a receiver is in range of more than one transmitter, the criteria for good reception include relative signal strength and total transmission delay. Relative signal strength describes the relationship of two or more transmitted signals, based on the location of the receiver, whereas total transmission delay is the elapsed time interval calculated from the moment that the signal leaves the studio site to the moment it reaches the receiver. This delay can differ from one transmitter to another, based on the signal path of the specific studio-transmitter link.
In a SFN implementation of an HD Radio system, one exporter can be used in combination with many exgines to improve coverage. The present inventors have observed a need for systems and methods that meet the following requirements for operation of single frequency networks in an HD Radio broadcast system.
With OFDM based systems such as an HD Radio broadcast system, the transmitters have to radiate not just the same but an identical on air signal. Thus, frequencies and phases of the sub-carriers have to be radiated to a very high tolerance. Any frequency offset between carriers in an OFDM system results in inter-symbol interference and a perceived Doppler shift in the frequency domain. For the HD Radio system the frequency offsets are expected to be within ˜20 Hz. In addition, the individual sub-carrier frequencies have to appear at the same time. Each transmitter has to radiate the same OFDM symbol at the same time so that the data is synchronized in the time domain. This synchronization depends in large part on the guard time interval, which governs the maximum delays or echoes that an OFDM-based system can tolerate. It also influences the maximum distance between transmitters. An OFDM receiver samples the received signal for a predetermined period of time at regular intervals. In between these sampling times (during the guard interval) the receiver ignores any received frequencies. For the HD Radio broadcast system, each OFDM symbol must be time aligned to within 75 μsec in order for the FM system to operate correctly. Preferably the alignment is within 10 μsec.
Another requirement is that the individual sub-carriers have to carry the same data for each symbol. In other words, the sub-carriers from the different transmitters must be “bit-exact”. This means that for each node in the SFN the digital information received at the transmit site from an exporter must contain the identical bits (i.e., MPS digital audio, program service data (PSD), station information service (SIS), and advanced application services (AAS) or other data must be identical). Moreover, the information must be processed by each exgine in an identical fashion so that the output waveform is identical for each transmission node of the network.
It is also desirable that the various pieces of equipment that comprise the network operate asynchronously, such that the equipment can come on or off line without requiring that the entire network be reset. The above described timing accuracies and “bit exactness” must be maintained during independent node restarts (i.e., each node in the SFN can be brought down and brought back up independently of all other nodes without affecting system performance). Each node of the SFN also must have the ability to adjust the transmission delay to account for propagation delays and to be able to tune the SFN.